Type Ia Supernova Spectrum

Supernovae fall into several distinct classes based on the elements present in the progenitor star that absorb light during the explosion and on the overall shape of the spectrum. Type II supernovae, for example, contain hydrogen. Type I supernovae, on the other hand, contain neither hydrogen nor helium even though those are the two most abundant elements in the universe. This is one reason why we think that SN Ia arise from stars that have undergone a lengthy process of stellar evolution before they explode—as stars evolve, hydrogen fuses into helium and helium into heavier elements. The spectrum shown above is of a type Ia supernova near maximum light. The features that distinguish it uniquely as type Ia, and therefore as a standard candle useful for tracing the expansion history of the universe, are the steep rise and slow fall in intensity as the wavelength increases and the deep dip in the spectrum at 6150 Angstroms due to the presence of silicon. (Unit: 11)